1. Field of the Invention
The present invention relates to a battery charger suited for charging a battery, such as a nickel metal hydride battery, which emits high heat while being charged.
2. Discussion of the Prior Art
Presently, a battery pack housing chargeable batteries which can be repeatedly used for the power supply of, for example, a power tool is used. A nickel cadmium battery is popular as a battery for the power tool.
The inventor of the present invention studied improving the performance of a power tool by employing a nickel metal hydride battery as a battery therefor. Although the nickel metal hydride battery can increase a capacity compared with a nickel cadmium battery, it generates high heat while being charged. If the temperature of the battery becomes high by the generated heat, the electrodes and separators of the cells within the battery deteriorate and battery life is shortened. Further, the nickel metal hydride battery is less resistant against overcharge than the nickel cadmium battery. If overcharged, the battery life is shortened. It is, therefore, necessary to avoid overcharge.
Considering this, the inventor of the present invention contrived a method of controlling charging current and detecting the completion of charge based on a map for determining a current value based on the absolute temperature of the battery and a temperature rise value. With this method, however, if the number of battery cells in a battery pack increases, a battery cell positioned on the end portion of the battery pack tends to be cooled and that on the central portion thereof is difficult to cool, with the result that the completion of charge cannot be detected based on the battery temperature. Furthermore, if the temperatures of the cells in the battery pack become increasingly irregular, the life of a high temperature cell is shortened and the high temperature cell is difficult to charge. Due to this, the capacities become unbalanced among the cells and it may occur that there is a cell charged 100% and one charged 90% (high temperature cell). Here, if the capacity of the battery pack is used up to 90%, some cells have a residual capacity of 10% and the others have a residual capacity of 0% (high temperature cell). The cells of 0% residual capacity are reverse-charged by those of 10% residual capacity and the battery life is considerably shortened.